Users should be able to create their own spa_dict.

With !12 (closed) merged, users can now read spa_dicts created elsewhere, but they can not yet create their own.

See this discussion for needed use-cases and proposals on how to implement them.

In short, we should add a way for users to:

• create an owned dictionary and provide it to a pipewire function (implemented by !22 (merged))
• create a static dictionary